This invention relates to the field of power conditioning systems. More specifically, to power supply current protection systems that function in the hiccup mode. Even more specifically, to hiccup-mode current protection systems employed by switched mode power converters.
Limiting of power supply output current or power under load fault conditions is a protective feature designed into many of today""s power conditioning equipment. Common methods to accomplish this usually fall into one of three classifications.
Constant current limiting prevents increases in current beyond a predetermined load current, Imax. Subsequent decreases in load impedance result in decreased output voltage with the output current being maintained at approximately Imax. With many types of switching regulators, constant current limiting is easily implemented using cycle-to-cycle peak current limiting.
Foldback current limiting causes the output current to decrease with any further decrease in the load impedance once the load current reaches Imax. Foldback limiting was originally developed to prevent thermal damage to output power devices in linear regulated power supplies. This concept has been applied to switching regulators, but can be both overly complex and unstable for many applications and does not offer advantages over hiccup mode current limiting.
Hiccup mode current limiting shuts down the power supply for a predetermined delay period when the current reaches Imax. If Imax occurs again, the shutdown and restart process repeats. With the proper design, hiccup design can be very effective at protecting both internal and external components and circuitry from thermal and overload damage.
For high power designs of a kilowatt or more, hiccup mode presents the best alternative of these three designs. Constant current limiting can be hazardous in these applications. If a constant current limit were set at 40 amperes and the load fault impedance were such as to drag the output voltage down to 25 volts, 1000 watts of power would be dissipated in this xe2x80x9csemi short.xe2x80x9d This is an unacceptable result that could cause a fire. Hiccup mode, properly designed, will deliver only a few watts of average power into a fault and is superior for high power designs.
Previous designs for current protection circuits using the hiccup mode, do not address the specific needs of a power supply environment. See Owen, et al., U.S. Pat. No. 4,481,553. Owen, et al. is incorporated herein by this reference. The Owen design is inferior for a power supply because it is designed to function in a power amplification environment and therefore does not provide the requisite high efficiency needed in the power supply environment. Further, and even more critical, the Owen circuit is designed to output a zero voltage once a high current condition is detected. This design feature does not disable the circuit from delivering current during a high current state because the design still functions as an active power amplifier. This result is not useful in a power environment where numerous redundant power supples may be connected in parallel.
What is needed is a hiccup-mode current protection circuit for switched mode power converters for high power applications that overcomes the deficiencies of the prior art designs.
It is therefore an object of this invention to provide a hiccup mode current protection circuit for switched mode power converters for high power applications.
This invention features an overcurrent protection circuit with a comparator for comparing an indicating voltage to a reference voltage and outputting a control signal to a disabling input where the indicating voltage exceeds a predetermined voltage threshold. The control signal also causes a switch to reduce the reference voltage to ensure that the comparator continues to output the control signal regardless of a drop in the indicating voltage. The control signal is maintained by a comparator for only a predetermined time period such that the switch eventually returns the reference voltage to its pre-reduction state, allowing the comparator to discontinue outputting the control signal if the indicating voltage is below the predetermined voltage threshold.